As is known, floating production units (FPUs) and, specifically, floating production, storage and offloading (FPSO) vessels, widely used for off-shore hydrocarbon production, are normally converted ships anchored permanently by mooring lines to the sea bed.
A mooring line normally comprises a first chain portion, which is fixed to a mooring station on the unit by a locking device; a cable portion (e.g. of synthetic material); and a second chain portion terminating with an anchor.
Normally, there are several mooring lines attached to respective stations variously arranged on the unit according to the required mooring configuration (e.g. spread mooring).
The normal procedure is as follows.
So-called “turn-down sheaves” with respective mooring line stop devices, known as “chain stoppers”, are set up on the unit, more specifically, along the sides of the ship; a chain portion, known as a “pilot” chain, is installed on each sheave; once the unit is in the mooring position, the mooring line, brought up to the unit by tenders, is connected to the pilot chain by a service chain portion, which attaches to the end link of the mooring line; the pilot chain is then connected to a winch to take up the pilot and service chains and tension the mooring line; and, once tensioned, the mooring line is locked by the respective stopper.
In one particularly advantageous solution described in International Patent Application n. WO2008/046874-A1, one main winch is used to tension all the mooring lines (as well as to handle other operating lines, such as production or extraction lines) by connecting the winch cable successively to the various chain portions for handling and/or tensioning.
The mooring method in the above International Patent Application, as well as others similar to it, are not without drawbacks.
A first of these lies in using winches. Normally, the end of the cable that attaches to the chain portions is defined by a cast head, known as a socket, which, as it runs through the sheave, tends to irreparably damage the cable.
A second drawback involves the orientation of the first link (of any chain portion) as it comes into contact with the sheave. Since it is practically impossible to ensure the first link of the incoming chain is in the ideal position to engage the sheave, and given also the amount of pull exerted by the cable when the chain reaches deck level, due to twisting of the cable, extremely hazardous situations may arise.
Systems more or less similar to the one described also pose the same problems.